1. Field of the Invention
The present invention relates to a small stepping motor which is reversible, that is, the motor shaft can step backward and forward.
2. Description of the Related Art
Stepping motors that are incorporated in small devices, for example, cameras, generally must be highly efficient, low in current consumption and small in size, since batteries are used as power sources therefor. Since reversible stepping motors need at least two sets of coils and stators, those according to the prior art have heretofore been arranged such that the stators and the iron cores are separated from one another or the stators are arranged in planes, one on top of the other.
A typical conventional small stepping motor of the type described above has a rotor comprised of a rotatably mounted and radially magnetized permanent magnet having a plurality of magnetic poles, a stator having a plurality of magnetic pole portions mounted to face the rotor, a coil wound around the stator, and a driving circuit for controlling the exciting current supplied to the coil. In operation, when either a plus (+) or minus (-) exciting current is supplied to the coil in response to an output signal from the driving circuit, magnetic pole portions are formed at the distal end of the stator, thus causing the rotor to rotate.
Thus, in a conventional small stepping motor of the type described above, an exciting current is supplied to the coil to generate magnetic pole portions in the stator, thereby rotating the rotor. In this prior art device, however, magnetic flux leaks out from the stator or the coil. For this reason, the magnetic flux density at the magnetic pole portions of the stator lowers and the magnetic coupling and repulsion forces acting between the magnetic pole portions and the magnetic poles of the rotor decrease, resulting in lowering in the level of rotational force on the rotor. It is therefore necessary, in order to raise the level of rotational force, to increase the exciting current supplied to the coil. This solution produces a rise in the current consumption rate. Accordingly, a battery of relatively large capacity must be incorporated into a small stepping motor, contrary to the above-described requirements for reduction in the over-all size and thickness of small stepping motors.